This invention relates to a control system for stabilizing the operation of an inverter-motor system during transients or disturbances, thereby greatly improving its dynamic behavior, and during steady state when the operating conditions are appropriate to cause system oscillation.
Inverter-motor systems have a tendency to oscillate at low frequencies under certain operating conditions, at which time the inverter appears as a negative resistance to the d-c bus over which the inverter receives its energizing voltage. This invention serves to make the inverter appear as a more positive resistance, thus reducing the tendency to oscillate.
In addition, the inverter-motor systems may have load torques which change abruptly, causing significant deviations from the desired steady state d-c bus voltage. This invention also serves to reduce the effect that the sudden load torque change has on the d-c bus voltage, thereby reducing the deviations from the desired d-c bus voltage.
The cause of the undesired bus voltage change is attributable to the presence of the filter, comprising a series-connected choke and a shunt-connected capacitor, which is usually included in the d-c power supply that produces the bus voltage. During normal steady state operation no average current flows to the filter capacitor and all of the current flows to the inverter and then to the motor windings. If there is a sudden drop, for example, in the load torque on the motor, the motor current abruptly decreases and the inductor current flows into the filter capacitor, charging it to a considerably higher voltage. Since the d-c bus is coupled across the capacitor, the bus voltage likewise increases significantly. The high bus voltage, in addition to deleteriously affecting the operation of the motor, may also destroy the switching devices (which may be transistors) in the inverter. To prevent such destruction, oversized switching devices are customarily employed having performance characteristics sufficient to withstand the highest bus voltage that could occur in the event of a very sudden load change.
The control system of the present invention achieves significantly improved dynamic performance and overcomes the instability problem by effectively regulating the d-c bus voltage to provide motor stability, while at the same time avoiding the necessity of oversizing the switching devices. With the present invention, the negative resistance of the inverter is made more positive to eliminate the tendency to oscillate. At the same time, the invention effects the way the motor torque changes with time to reduce the d-c bus voltage deviation from the desired value.